I wouldn't call this my dream project, but since we're throwing ideas around, I might as well post this here.
So, we already have a Dracula origin story. Sort of. However, I think LoI only tells us the beginning of that story: how Mathias became a vampire. The epilogue then quickly catches up with the situation shortly before 1476 with Mathias calling himself Maou and Dracula (in the Japanese version). What I had in mind was a game which depicts the transition from Mathias to Vlad Tepes and finally Dracula. This has been proposed a lot of times already but I have a slightly different take on things, so hear me out.
First a quick clarification, I'm going with the story that Msthias doesn't become the Dark Lord until after Lisa's death (which has been stated before in a source) as well the idea that he doesn't take on the name "Devil" (Dracula) until that time as well (which isn't quite canon but feels like the most appropriate time to me).
But how would such a story work? I don't playing as the villain would be fun and appropriate for a Castlevania game at all. Lords of Shadow did it but that story is really awful. So I would propose something that would probably get some protest at first: make Dracula the legitimate hero of the story. You would root for him in the same way you would as for any other protagonist in the series.
But then say: "That wouldn't work at all because in LoI Mathias renounced God and became a vampire". Which is a fair point, but in the Encyclopedia of Castlevania it does say "Dracula once more regained his human heart" after he fell in love with Lisa. So I would use that as my justification for why it would OK to consider Dracula the good guy. Not necessarily that he is a goody two-shoes, but more like a slightly more roughed version of Alucard, who has something legitimately terrible in his past, but with still a chance of redeeming himself.
What I think would be interesting to explore would be how Dracula 1) obtained his castle 2) became a Count and 3) became "the protector of the land". My idea for the setting of the game would Wallachia roughly in the 1440s or 1450s starring "Vlad Tepes" (perhaps just Vlad) and Lisa, before they married and had Alucard. Also, I imagine Dracula appears young in this game because pretty much all protagonist in the series appear that way. He probably wouldn't dress like his traditional self yet, so he would look more like Alucard, I guess?
As for the conflict/antagonist, as I mentioned before, Dracula wouldn't be the Dark Lord yet, so that would be someone else (I'll leave the identity to the imagination). I don't think it would be a good idea to make this character a large scale treat because that kind of undermines what Dracula eventually does, so more like Walter.
Let's say this character is causing some trouble locally. The Belmonts aren't around because they are exiled. Vlad Tepes is requested to take this character down. But by who and what would be his motivation be for doing this? I think my "Akumajo Lisa" idea works well. Lisa is the local Countess and last descendant of a knight family which was originally tasked with defending the land, but has recently fallen in decline. So she tries recruiting other knights who could take on this local treat. Since Vlad Tepes was a knight and brilliant tactician in his former life, it would make sense for him character-wise to be recruited. His motivation for helping would be because Lisa is the spitting image of his former wife.
Then the game proper begins. Perhaps the villain ends up taking over Lisa's castle in the story and turns it into the Demon Castle we all know, so it would still be like a regular Castlevania game. Vlad fights only with dark-aligned equipment. I think the "Demon recruitment" system I mentioned before would work very well in a setting like this.
The epilogue would be Vlad and Lisa marrying, him inhereting the castle and title of Count, and with the birth of Alucard.